Primary sources are the sources of information that are closest to the topic of your study.  You might think of them as the raw materials of scholarship.  If you are studying a historical topic, letters, diaries, newspaper articles from the time, eyewitness accounts, interviews, and oral histories might be primary sources.  Publications are usually secondary sources, but they can be primary if they are written by a person you are studying.  Datasets can be primary sources as well, particularly for behavioral or empirical research.

The WMU Libraries offer many databases that link to digitized primary sources.  You might also look at the websites of large research libraries, such as those found at other universities, or government libraries like Library of Congress, or other national libraries of other countries, such as the British Library, or Library and Archives Canada, etc.  Museums frequently offer digitized images of the items in their collections on their websites.